A PLC (Programmable Logic Controller) is a control device that is used to control an automated machine in a factory or the like. Such a control device is connected to various devices (apparatuses) such as a sensor and an actuator, so as to control operations of these devices by executing a control program. The process of controlling operations of these devices is hereinafter referred to as “device control process”.
In recent years, a technique of performing integrated information management in cooperation with control devices such as the PLC and an external information system has been put to practical use. As this kind of technique, MES (Manufacturing Execution System) and ERP (Enterprise Resource Planning) can be mentioned.
In a large scale plant such as a water treatment plant or a chemical plant, high-speed and large-capacity data is handled. In such a large scale plant, a monitoring control system referred to as “SCADA (Supervisory Control And Data Acquisition)” is generally used. As functions of the SCADA, a data collection function and a user interface function can be mentioned. The data collection function is a function of collecting device data from devices such as a sensor in a plant and storing the collected device data in a database. The user interface function is a function of displaying device data on a display screen of a monitoring terminal and receiving data manipulation from the user. The user can monitor the device control device in the plant and perform required data manipulation, based on the data displayed on the display screen of the monitoring terminal.
As a control device that transfers data between an external information system and the device itself, the following devices have been known.
Patent Literature 1 discloses a distributed control device. This distributed control device includes a main processor for device control and a communication processor for data communication. Owing to this configuration, even if the distributed control device receives large-amount data acquisition requests from a high-order system, influences thereby on the device control process can be reduced.
Patent Literature 2 discloses a technique for remotely controlling a PLC through the Internet. The PLC receives a data transmission request related to a user program from a remote control computer. In response to the transmission request, the PLC transmits the user program and control information in a database to the remote control computer.